Do not bail out when passed a flags string with length > 5, use a
meaningful named constant instead.

Found by https://github.com/tc39/test262/pull/997#issuecomment-296963675

BUG=v8:6300

Review-Url: https://codereview.chromium.org/2841633004
Cr-Commit-Position: refs/heads/master@{#44834}